text: There are three known thrombin receptors (ThrR), termed PAR1, PAR3 and PAR4 (PAR for protease-activated receptor).
  G-protein-coupled receptors that are responsible for the coagulation effects and responses of thrombin on cells are known as protease-activated receptors, or PARs. These receptors are members of the 7-transmembrane g protein-coupled family of receptors, however, their method of activation is unique. Unlike most G-protein-coupled receptors, PARs are irreversibly activated by proteolytic mechanism and therefore, are strictly regulated.
  Thrombin is an allosteric serine protease that is an essential effector of coagulation that is produced at sites of vascular injury and plays a critical role in cellular response to blood-related diseases. It binds to and cleaves the extracellular N-terminal domain of the receptor. A tethered ligand corresponding to the new N-terminus, SFLLRN, is then unmasked, binding to the second extracellular loop of the receptor and activating it.